Hydrozoa

Pronunciation: hi-dro-zo'a

Definition: A class of coelenterates or jellyfish, including Hydra, a freshwater polyp, Physalia, the “Portuguese man-of-war,” Millepora, a stinging coral, and the sea wasps, Chironex heckeri and Chiropsalmus quadrigatus, the stings of which can cause severe wheals, pain, skin necrosis, and occasionally, rapid death from respiratory and cardiac depression.